A deficiency of a hitherto unrecognized factor necessary for hemostasis has recently been reported in two unrelated families in the San Diego area. The affected patients have a moderate bleeding diathesis associated with a prolonged activated partial thromboplastin time and in accordance with precedence the deficient factor was designated as Passovoy after the patient's surname. We propose to improve the current assay procedure which is relatively insensitive; purify the clotting factor using conventional biochemical techniques, and determine its mode of action.